Problem: Add. $\begin{aligned} 21& \\ 21&\\ 37&\\ \underline{+19}& \\ \end{aligned}$
Answer: First, let's add $21 + 21$. $$ $$ $$ ${+}$ $$ $$ $$ ${=}$ $$ $$ $$ $$ $$ $21} + {21} = 42$ Now, let's add $37$ to $42$. $$ $$ $$ $$ $$ ${+}$ $$ $$ $$ $$ $$ ${=}$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $42+ {37} =79$ Last, let's add $19$ to $79$. $$ $$ $$ $$ $$ $$ $$ $$ $$ ${+}$ $$ $$ $$ ${=}$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $79 + {19} = 98$ $\begin{aligned} 21& \\ 21&\\ 37&\\ \underline{+19}& \\ 98& \end{aligned}$